Subfloor complete.
Built the subfloor over two days. This is a rectangle of 2x2s (ripped 2x4s) with additional 2×2 cross-braces. The subfloor is 10′x4′ and sheets of plywood are only 8′ long. Under the subfloor, there’s a single 2×4 to handle the gap and shorty plywood.
Painted two coats of green wood preservative on the bottom and sides so the road splatter doesn’t rot it out from underneath.
It will be bolted through the chassis. Here’s a pic with it resting on top:
